Proliferation of nonlinear loads attached to AC power supply systems has been increasing in recent years. Such a nonlinear load is exemplified by a motor controller (MC) for motor driven loads as commonly used in the aerospace industry. Most of this equipment uses a six-pulse rectifier front-end to convert alternating current (AC) power produced by a generator to direct current (DC) power. Such nonlinear loads cause the rectifiers to inject harmonic current due to their nonlinear behaviour. This extraneous harmonic current can cause problems, such as harmonic resonances, increased losses and overheating of the electromagnetic components on the bus, such as transformers and motors.
A MC is used in conjunction with an AC dynamoelectric machine to provide variable and controllable speed for a multiplicity of applications. It converts a DC power source to polyphase AC of suitable power and frequency to drive the AC dynamoelectric machine for such applications.
In aircraft, a MC has uses for both low power and high power applications. One high power application is main engine starting, which may be accomplished with a MC and a dynamoelectric machine that functions as a starter motor and a main electrical power generator. Another such aeronautical high power application is the use of a MC for an electric motor driven hydraulic pump (HP). In aircraft, it is common practice to use the main engine starting MC to serve another function after the engine is started, such as controlling other motor driven loads, also requiring high power ratings.
MCs with a six-pulse rectifier front end tend to generate significant harmonics of their operating frequencies, which may be on the order of 1000 Hz or more in aeronautical applications. The most significant harmonics tend to be the 5th, 7th and 11th. These reflect back from the MC into the AC bus and may adversely affect the operation of other equipment.
Use of tuned passive inductive-capacitive (LC) filters to control extraneous harmonic currents is limited in variable frequency applications commonly used in the aerospace industry. In addition, the presence of LC filters on the AC bus may form series and parallel resonances with the loads, resulting in amplification of harmonic currents. Use of 12-pulse and 18-pulse rectifier front-ends, as well as active front-ends, may be used to alleviate such harmonic currents, but they generally increase weight, size, and cost of the motor drive.